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Experiment using a potentiometer

  1. Oct 11, 2016 #1
    1. The problem statement, all variables and given/known data
    In this experiment, we're trying to study the characteristics of a light bulb. Here are the steps outlined for us:
    1. Use a potentiometer to vary the voltage across the bulb measured by the voltmeter V. Collect data of V-values and corresponding I'-values from the ammeter over the voltage range 0-12 V.

    2. Remove the bulb from the circuit and repeat (1) using identical voltage values. The new current values i represent the current drawn by the voltmeter itself during part (1). Tabulate I' and i with corresponding V, then calculate the corrected values I = I' - i, tabulating these also.

    2. Relevant equations
    R= v/I and P=VI

    3. The attempt at a solution
    We are able to set the experiment for part (1) and we get the values required. For the second part, the only problem is, we are using a rheostat as a potentiometer so we've connected all three terminals of the rheostat. The bottom two terminals of the rheostat have been connected to the D.C power supply and then another connection was made to the voltmeter from the wiper to one of the bottom ends as well and we got ourselves a potential divider.

    Now the only concern is where do we place the ammeter to be able to measure the current? We've tried different positions but most of the time, either the ammeter would work alone or the voltmeter, never together!
     
  2. jcsd
  3. Oct 11, 2016 #2

    BvU

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    Since the instructions mention 'the current drawn by the voltmeter itself", it seems they want you to measure the current through a palallel circuit of bulb and voltmeter:

    upload_2016-10-11_13-42-48.png
     
  4. Oct 11, 2016 #3
    But in the second part they mention that we have to remove the bulb?
     

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  5. Oct 11, 2016 #4

    andrevdh

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    Unplug the two plugs at the left end of the rheostat, but do not separate the plugs from each other (that is the black plug from the voltmeter and the red plug of the ammeter in the photo, they are plugged into the slider of the rheostat).
    Now unplug the black plug of the ammeter at the other side of the rheostat (it is plugged into the other two red plugs).
    Insert this plug where you removed the previous two plugs - that is into the slider of the rheostat.

    The ammeter will now measure the current drawn by the voltmeter.
     
  6. Oct 11, 2016 #5
    Where do I plug in the two plugs (the black plug from the voltmeter and the red plug of the ammeter) that I removed from the left side of the slider?
     
  7. Oct 11, 2016 #6
    Ah I think I might have understood. As per my understanding, there's a direct connection from the positive end of the ammeter to the negative end of the voltmeter directly? And the negative end of the ammeter is connected to the left end of the slider? Is that correct?
     
  8. Oct 11, 2016 #7

    cnh1995

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    In your above image of the setup, the ammeter looks to be connected in parallel with the voltmeter. This is shorting the voltmeter out. I think this is why both the meters do not work at the same time. I believe andrevdh's advice in #4 will solve your problem.
     
  9. Oct 12, 2016 #8
    So I tried it again today with the corrections mentioned and the ammeter doesn't show any deflection at all.
     
  10. Oct 12, 2016 #9

    cnh1995

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    The ammeter should be in series with the voltmeter if you want to measure the current drawn by the voltmeter.
    That might be because of high voltmeter resistance. Measure the voltmeter resistance (it would be in the range of a few hundred kilo-ohms). You can theoretically calculate the voltmeter current using I=Vapplied/Rvoltmeter and see if it can be measured by the ammeter. What is the smallest current you can measure with the ammeter?
     
  11. Oct 12, 2016 #10

    andrevdh

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    potentiometer with steps.jpg
    1. Remove the two plugs from the slider connection, but do not separate them from each other.
    2. Remove the black ammeter wire from this point and insert it where you removed the previous two plugs, that is insert it into the rheostat slider plug.

    Did you switch the ps on?
    Any deflections on the meters?
    If not you might have blown the ammeter or its fuse :nb) or maybe the power supply's fuse.
    To check the ps remove any wires from it and use the voltmeter to measure its output.
    If it is fine then the ammeter is probably blown.
    You can test if the ammeter is still functional by restoring the light bulb circuit, alternatively get help from the person in charge.
     
    Last edited: Oct 12, 2016
  12. Oct 12, 2016 #11
    I quickly looked at the setup picture without following every lead.. It looks like the Voltmeter and ammeter are hooked up to the same points or nodes, this is not correct because the ammeter meter must be hooked in series with the branch or circuit to measure current and the volt meter must be hooked up in parallel across a branch or circuit to read the voltage!

    Both meters seem to be hooked up in parallel.

    Ammeters should have very low internal resistance and voltmeters shall have very high internal R's.

    Again, the V meter and A meter cannot be hooked up the same, they must be hooked up in parallel and series..

    Also, I have never seen a voltmeter and ammeter load down a equivalent DC circuit like yours if hooked up correctly, IOW, using both meters at the same time should not affect your measured results in this particular config..I doubt at this level they would want you to measure or calc. internal R's and such..Ya' never know though, there are some crazy teachers out there, I was one of em'... :redface:
     
    Last edited: Oct 12, 2016
  13. Oct 12, 2016 #12
    Yes I did switch on the power supply, only the voltmeter needle deflected, not the ammeter. I took the ammeter out and tested it in another circuit and it was working. I even replaced it with a new ammeter and it didn't work either. The power supply was working as well (checked it separately). :/
     
  14. Oct 12, 2016 #13
    Smallest current it can measure is 0 A. The range is 0-12 A.
     
  15. Oct 12, 2016 #14

    cnh1995

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    No.. I meant its resolution. If the actual current is 1mA, is the meter able to read that small value? Or does it show 0? What is the value of the smallest division on the ammeter scale?
     
    Last edited: Oct 12, 2016
  16. Oct 12, 2016 #15

    andrevdh

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    Good point cnh. She probably needs a more sensitive ammeter then since the current might be rather small due to the large resistance of the voltmeter.
    The one in the photo seems to measure up to 5A?
     
  17. Oct 12, 2016 #16
    Ah yes sorry, it's maximum current is 5A and the smallest division is 0.1 A. I plugged in a multimeter for measuring in mA but even the multimeter was showing 0.
     

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  18. Oct 12, 2016 #17

    cnh1995

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    If the current comes out to be less than 100mA, the ammeter will read 0. (Neglect ammeter resistance here since it is very small).
     
  19. Oct 12, 2016 #18
    So what should I do then? How can I read it with a multimeter?
     
  20. Oct 12, 2016 #19

    cnh1995

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    Well, a digital multimeter should be able to read currents in mA. If it is showing 0, then the current is really small (in microamps).

    I believe this is how you have connected your setup (values may differ). Try increasing supply voltage (if the experiment permits..). It all depends on the voltmeter resistance.
    Screenshot_2016-10-12-18-56-03.png
     
  21. Oct 12, 2016 #20

    BvU

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    So you found some currents and are able to draw a plot of I (vertical) versus V (horizontal) ? And the light bulb actually functioned (gave more light with higher voltage) ?

    Analog meters like the ones you are using are in fact current meters with a sensitivity of 50 ##\mu##A full scale. So a voltmeter with 10 V full scale is made out of such a device by putting it in series with a 200 000 ##\Omega## resistor. At 10 V there is a current of 50 ##\mu##A that you can't measure with an ammeter with 5 A full scale.

    (The ammeter is a similar meter with a very small parallel resistance: e.g. if the 50 ##\mu##A meter has a resistance of 100 ##\Omega## the voltage drop over the meter is 5 mV at full scale; so they place a parallel resistance of 0.5 ##m\Omega##)

    To check these numbers you could use a multimeter to measure the resistance of the voltmeter.
     
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